GROWTH OF CLOUD DROPS BY CONDENSATION 
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Fie. 2—Assumed variation of 
TIME (MINUTES) 
vertical velocity 
and corresponding 
changes of temperature, pressure, and height with time, Cumulonimbus 
Case; at top, temperature difference between cloud and environment 
height and the corresponding changes of tempera- 
ture, pressure and height with time. The Type A 
nucleus distribution was assumed. 
The temperature difference between the cloud 
and the environment, computed from the vertical 
acceleration, is shown at the top of Figure 2. In 
the lowest part of the cloud the assumed accelera- 
tion corresponds to only a small fraction of one 
degree, and even in the upper part, in which the 
vertical velocity increases rapidly, it corresponds 
to the cloud being only about one-half degree 
centigrade warmer than the environment. 
In the trade-wind Cumulus model the vertical 
velocity distribution was based on the observa- 
tions by Malkus [1954]. The temperature and 
relative humidity at 1000 mb were assumed to 
be 301.7°K and about 75%, and the cooling was 
assumed to be adiabatic. Figure 3 shows the as- 
sumed variation of vertical velocity and the 
corresponding changes of temperature, pressure 
and height for this case. For the trade wind Cumu- 
lus case the Type B nucleus distribution was as- 
sumed. 
The assumed acceleration corresponds to a 
larger temperature difference between cloud and 
environment in the lower part of the trade wind 
Cumulus than in the Cumulonimbus (see upper 
portion of Fig. 3). The deceleration near the top 
corresponds to the cloud becoming 1°C colder 
than the environment. 
Properly, in computing the cooling the release 
of latent heat should be computed directly from 
the amount of liquid condensed at each step. J. 
E. MeDonald has pointed out that in the early 
stages of drop growth the difference between this 
and the assumption of saturation adiabatic equilib- 
rium might lead to significant differences in the 
cut-off point between the nuclei which grow to 
cloud drops and those which remain small. In a 
future computation it is planned to examine this 
point. For the computations presently being re- 
ported the decrease in environmental tempera- 
ture was assumed to be that resulting from satura- 
tion adiabatic cooling. 
The Stratus cases—The variation of the relative 
humidity during the isobaric temperature decrease 
(Stratus Case A) is shown in the upper portion 
of Figure 4, and the variation in size of the differ- 
ent droplet groups in the lower portion. Satura- 
tion with respect to a plane water surface is 
reached after about 2650 sec. Until about 2400 
sec the hygroscopic nuclei all increase slowly in 
size; from then to the time of maximum super- 
saturation, about 2700 sec, all groups grow more 
and more rapidly; after that the degree of super- 
saturation decreases, and the smallest two groups 
of drops decrease in size, while the groups growing 
on nuclei of 0.1 micron or larger continue to grow 
rapidly. By 2800 sec all the growing groups exceed 
four microns, while the non-growing nuclei re- 
